(1) Field of the Invention
The present invention relates to a fixing device for fixing an unfixed image formed on a recording sheet onto the recording sheet by heating thereof, and to an image forming apparatus provided with the fixing device.
(2) Description of the Related Art
In image forming apparatuses such as printers or copiers adopting the electrophotographic method, typically a toner image corresponding to a piece of image data is transferred onto a recording paper sheet or a recording sheet such as an OHP sheet, and the unfixed toner image is then fixed onto the recording sheet by the fixing device. The fixing device fixes a toner image onto a recording sheet by heating the toner image and pressing it against the recording sheet.
As the structure of such fixing device for image forming apparatuses, a structure using a heating fixing belt is known, the heating fixing belt being provided with a resistance heating layer that emits heat as the current flows. For example, Patent Literature 1 (Japanese Patent Application Publication No. 2004-281123) discloses a fixing device in which a pressing roller is pressed against a heating belt (the heating fixing belt) provided with the resistance heating layer. In this fixing device, while the heating belt in the heating state moves in a circling motion, the recording sheet is pressed against the heating belt by the pressing roller, which enables the toner image to be heated and fixed onto the recording sheet.
The resistance heating layer provided in the heating belt is typically made of an insulation material in which an electrically conductive material is added so that the resistance heating layer has a predetermined electric resistivity, wherein the insulation material is, for example, an insulation ceramic or a heat-resistant resin, and the electrically conductive material is, for example, an electrically conductive ceramic, an electrically conductive carbon, or metal powder. The heating belt is provided with power supply terminals at both sides thereof in the width direction, and the surfaces of the power supply terminals on the inner circumference side of the belt are exposed. Power is supplied to between the two power supply terminals when the power supply terminals are in contact with roll or bar electrodes. This causes the resistance heating layer to be heated.
Also, Patent Literature 2 (Japanese Patent Application Publication No. 2007-272223) discloses a fixing device provided with a heating fixing belt and a pressing roll, the heating fixing belt including a heating layer. In this fixing device, a belt holder is provided in a circling motion area of the heating fixing belt, the belt holder includes two electrodes which face side edge portions of both sides of the heating layer in the width direction, and the respective electrodes are caused to be in sliding contact with the side edge portions of both sides of the heating layer.
Furthermore, Patent Literature 3 (Japanese Patent Application Publication No. H6-348155) discloses a fixing device in which a fixing nip is formed between a heating medium and a pressing elastic roller that are pressed against each other, wherein the heating medium is in a sleeve or belt shape and is composed of: an electrically conductive supporting layer; and a heating layer stacked on the supporting layer. The fixing device includes a roll or bar electrode provided in parallel with the rotational axis of the heating medium, and the electrode and the heating layer are brought into contact with each other, which causes the electric current to flow between the electrode and the electrically conductive supporting layer, thereby heating the heating layer.